1. Field of the Invention
Embodiments of the invention relate to a method for fabricating a semiconductor memory device. In particular, embodiments of the invention relate to a method for measuring critical dimensions of a pattern using an overlay measuring apparatus.
2. Description of Related Art
In general, the fabrication of a semiconductor memory device comprises depositing materials to form thin films having various functions on a wafer surface and patterning the deposited materials to form various geometric circuit structures. Unit processes for the fabrication of a semiconductor memory generally include: an impurity ion implantation process for implanting impurity ions of Group 3B (for example, boron (B)) or 5B (for example, phosphorous (P) or arsenic (As)) into a semiconductor substrate, a thin film deposition process forming a material layer on a semiconductor substrate, and an etching process for forming the material layer into pattern. Additional unit processes include a process for depositing an interlayer insulating layer on a semiconductor substrate, a chemical mechanical polishing (CMP) process for planarizing the top surface of a wafer by polishing the wafer surface (after depositing the interlayer insulating layer) in order to get better step coverage (and the like) on the wafer, and a wafer and chamber cleaning process for removing impurities.
Semiconductor device technology has been advancing relatively quickly along with the relatively fast development of information and communications technologies and the relatively fast growth in the popularity of information technology media such as computers. Further, because semiconductor devices operating at relatively high speeds and having relatively high storage capacities are desired, the degree of integration of semiconductor devices progressively increases. In addition, as the degree of integration of semiconductor devices increases, the respective size of each element (i.e., unit element) of a semiconductor memory cell is gradually reduced. Accordingly, a multi-layer structure, in which all necessary patterns may be disposed in region having a limited area, is increasingly being used in achieving relatively high degrees of integration.
A double layer process and a stack transistor process are examples of processes that are widely used to form a multi-layer structure. The double layer process connects a number of metal layers using metal via contacts. The stack transistor process forms two or more transistors in a vertical structure on the same vertical line of a semiconductor substrate.
In a semiconductor memory device, the pattern density of a memory cell region of the device is much higher than the pattern density of a peripheral circuit region of the device. Thus, as the degree of integration of a semiconductor device increases, the step coverage of a material layer gets worse because of the step (i.e., difference in height) between adjacent patterns. Moreover, as the degree of integration of a semiconductor memory device increases, the resolution of a photolithography process deteriorates, making it relatively difficult to form a pattern with an accurate profile. In addition, as the degree of integration of a semiconductor device increases, insufficient process margins causes misalignment in the semiconductor device. Therefore, for a photolithography process, which is a core process in the fabrication of a semiconductor device, it is essential to check a degree of overlay (that is, the overlay extent) between a previously-formed lower pattern and an upper pattern to be formed in a present processing step and to accurately control critical dimensions of a pattern in accordance with the design rules in order to substantially reduce the misalignment between the patterns.
FIGS. 1A and 1B illustrate a conventional process for forming (i.e., opening) a fuse box. In FIG. 1A, a fuse pattern 12 is formed on an interlayer insulating layer 10 that was formed on a semiconductor substrate to form a relatively flat surface on (i.e., to planarize a top surface of) the semiconductor substrate. Fuse pattern 12 is a contact that transfers a voltage applied from the outside to an internal transistor and may be formed from a metal material such as, for example, aluminum or copper, and the like.
In addition, a non-photosensitive polyimide layer is formed on interlayer insulating layer 10 as a passivation layer 14 to protect fuse pattern 12. After photoresist (PR) is applied onto passivation layer 14, conventional exposure and development processes are performed to form a mask pattern 16 used in etching passivation layer 14.
Then, a conventional photolithography process is performed on the resultant structure on which mask pattern 16 has been formed. As a result, as illustrated in FIG. 1B, passivation layer 14 is etched such that a portion of a surface of fuse pattern 12 is exposed and a fuse box 18 is thereby formed.
When passivation layer 14 is a non-photosensitive polyimide layer, mask pattern 16 must be formed on passivation layer 14 in order to etch passivation layer 14. However, photosensitive polyimide (PSPI) may be used to form passivation layer 14 rather than the non-photosensitive polyimide. When PSPI is used to form passivation layer 14, mask pattern 16 does not need to be formed on passivation layer 14 in order to etch passivation layer 14. So, when PSPI is used to form passivation layer 14, passivation layer 14 may be etched without having to perform the process for forming mask pattern 16 using photoresist.
FIG. 2 illustrates a process for forming a fuse box using PSPI. In FIG. 2, an interlayer insulating layer 100 is formed on a semiconductor substrate (not shown) to form a relatively flat surface on a top surface of the semiconductor substrate on which a transistor is formed. A fuse pattern 102, which functions as a contact to transfer a voltage applied from the outside to an internal transistor, is formed on interlayer insulating layer 100. Fuse pattern 102 may be formed from a metal material such as, for example, aluminum or copper, and the like.
In addition, a PSPI layer is formed on interlayer insulating layer 100 and fuse pattern 102 as a passivation layer 104 to protect fuse pattern 102. Thereafter, passivation layer 104 is partially etched to form a fuse box 106 to expose fuse pattern 102.
When passivation layer 104 is a PSPI layer, passivation layer 104 may be etched without performing the additional process of forming a mask pattern on passivation layer 104. That is, because the PSPI layer functions as a photoresist, when passivation layer 104 is a PSPI layer, substantially the same result may be obtained by etching passivation layer 104 without applying photoresist as by performing a conventional photolithography process using the photoresist. FIGS. 1A and 1B illustrate a method for forming fuse box 18, wherein the method includes forming a non-photosensitive polyimide layer as passivation layer 14, forming mask pattern 16 from photoresist, and etching passivation layer 14 using mask pattern 16 to form fuse box 18. However, FIG. 2 illustrates a process for forming fuse box 18 that includes forming a PSPI layer as passivation layer 104, and forming fuse box 106 through one process of etching passivation layer 104 using a photolithography process. Therefore, the method illustrated in FIG. 2 comprises fewer processing steps than the method illustrated in FIGS. 1A and 1B. Also, the refresh characteristic is improved through low temperature hardening.
To perform the aforementioned processes for forming a fuse box, the size of the fuse box needs to be measured. A scanning electron beam microscope (SEM) is conventionally used to measure the size of the fuse box. However, when the passivation layer is a photosensitive polyimide layer, the SEM equipment generates fumes that inhibit the functionality of the SEM equipment.